The present invention is directed to a method of producing cyclic carbonic acid esters. More particularly, the present invention is directed to achieving a low-cost method of producing trimethylene carbonate.
Cyclic carbonic acid esters are used, for example, as building blocks of potentially biodegradable polymers. A particular cyclic carbonic acid ester, trimethylene carbonate (trimethylene carbonate or 1,3-dioxan-2-one), may be used in a variety of applications, such as for surgical stitching material, vessel implants, and apparatus for osteo-synthesis. Trimethylene carbonate is a desirable monomer to use because of its property of not decreasing in volume on polymerization.
Trimethylene carbonate may be used as a monomer in the synthesis of poly(trimethylene carbonate) polyols, which are used in flexibilizing acrylic melamine coatings. Trimethylene carbonate may also be used to make surgical sutures and modified polyurethane elastomers. Poly(trimethylene carbonate) polyols improve both ambient and low temperature flexibility and reduced the viscosity of urethane coatings formulated with selected commercial acrylic polyols.
For industrial production of trimethylene carbonate, it would be desirable to find a method of synthesis yielding cyclic carbonates in high yields by a relatively simple industrial process. Numerous methods are known for producing carbonic acid esters, such as trimethylene carbonate. For example, the transesterfication of diethylcarbonate with 1,3-propanediol in the presence of sodium or sodium methoxide to obtain trimethylene carbonate is one of the oldest methods of production (W. H. Carothers et al., J. Am. Chem. Soc., 52 (1930) 322), but the purity of the product obtained is not sufficient for use in polymerization reactions, which results in a lower grade product. In addition, the low yield makes this method unattractive for industrial use.
U.S. Pat. No. 5,212,321 to Muller et al. discloses a method for producing trimethylene carbonate where 1,3-propanediol is reacted with diethylcarbonate in the presence of zinc powder, zinc oxide, tin powder, tin halide, or an organo-tin compound, at an elevated temperature. However, the Muller et al. process is very expensive as the process, the separation, isolation, and disposal of residues and catalysts or catalyst material are time-consuming and expensive.
U.S. Pat. No. 5,091,543 to Grey discloses a method of preparing five- and six-membered cyclic carbonates. The method involves reacting a 1,2- or 1,3-diol with an acyclic diester of carbonic acid in the presence of a catalyst selected from alkylammonium salts, tertiary amines, and ion-exchange resins containing alkylammonium or tertiary amino groups. Cyclic carbonates free of polycarbonate by-products are obtained in high yields. However, the Grey process is also very expensive, as the process requires the use of reactors made from materials of construction that will not corrode when exposed to the halide ions in the process. Isolation and disposal of residues and catalysts are also time-consuming and expensive.
Another process used to prepare trimethylene carbonate involves reacting 1,3-propanediol with urea in the presence of zinc-based catalysts. This type of process is described, for example, in Japanese Patent Nos. 7-330686 and 7-330756. The process requires expensive and time-consuming isolation, recovery, and recycling of the catalysts.
Trimethylene carbonate has also been made by reacting 1,3-propanediol with ethylchloroformate while using two equivalents of triethylamine (Toshiro Agriga et al., Macromolecules, 30 (1997) 737). However this method produces trimethylene carbonate in low yield and requires large amounts of triethylamine.
The vapor-phase reaction between phosgene and an alcohol is known to form the corresponding chloroformate (Saunders et al., J. Am. Chem. Soc., 87 (1965) 2088). Continuous processes for the formation of chloroformates from phosgene and alcohols are disclosed in Japanese Patent Nos. JP 51-043719 and JP 51-043721.
There remains a need for a low-cost method for producing trimethylene carbonate. A low-cost method desirably involves production of trimethylene carbonate in relatively high yields with reduced expenses for clean up and/or recycling or disposing of residues and/or catalyst material. A combination of several or all of these desirable features would be even more desirable.
The present invention is directed to a novel method of synthesizing trimethylene carbonate. The method comprises reacting 1,3-propanediol and phosgene in vapor form while providing a combination of temperature and pressure at which trimethylene carbonate boils or is in a vapor phase, and providing a residence time at those conditions sufficient to react at least some of the 1,3-propanediol and phosgene to trimethylene carbonate.
An advantage of the method of the present invention is that it does not require the use of catalysts and their associated expense of recovery and recycling or disposing of catalyst residues. Although not required, the use of catalysts is not precluded in the method of the present invention if desired.